1. Field of Invention
The invention pertains to stereoscopic imaging systems, and more particularly to stereoscopic image capture endoscopes.
2. Background Art
The field of minimally invasive surgery (e.g., laparoscopic surgery) requires increasingly smaller, increasingly mobile stereoscopic imaging systems. Stereoscopic endoscopes are typically mounted at the distal end of rigid shafts that extend through a cannula so as to image a surgical site during, e.g., robot-assisted surgery.
To get acceptable stereoscopic imaging without causing viewer fatigue or eyestrain, the images of the target object viewed by the two imaging systems should match in at least the following alignment parameters: (1) image location along the horizontal axis; (2) image location along the vertical axis; (3) image rotation; (4) image scale; (5) geometric distortion; (5) focus at the image center; (6) focal shift along the horizontal axis; and (7) focal shift along the vertical axis. The tolerable errors in the matching between the two images in a stereoscopic pair depend to some extent upon the display and viewer, but in general are much more stringent requirements than exist for monocular viewing. In addition, except for the image location, mismatches in the other parameters are difficult to correct for in image post-processing without introducing imaging artifacts.
While these parameters are all affected to some degree by the positions of the optical elements in the imaging system, they are also affected by the accuracy of the mounting of the two image sensors conventionally used in a stereoscopic endoscope with respect to each other. Taking one of the sensors as a reference, the position of a second, separate, sensor has six degrees of freedom in its mounting: three of translation and three of rotation. Errors in two of the translation axes between the sensors affect the relative horizontal and vertical positions of the viewed images, while errors in the third translation axis, the axis perpendicular to the sensor surface, affects both the image scale (if the objective is not telecentric in image space) and focus. Errors in rotation between the two image sensors, around the axis perpendicular to the sensor surface, directly affect image rotation and cannot always be corrected by alignment of the optics, while rotation errors about the other two axes affect the focal plane shifts across the imaging field.
In three dimensions, a rigid body (e.g., an optical image sensor chip) has six degrees of freedom: moving up and down (heaving), moving left and right (swaying), moving forward and backward (surging), tilting up and down (pitching), turning left and right (yawing), and tilting side to side (rolling). With two separate image sensors there are a total of 12 degrees of freedom that must be controlled when mounting the two sensors to the optical train. For example, if two physically separate sensors are used, then each sensor must be aligned with its respective optical train and additional image processing (e.g., to compensate for rotation) is required in order to align the captured left and right images with each other to present the stereoscopic view to a person viewing the images.